1. Field of the Invention
The present invention relates generally to a process of continuous casting of steel and, more particularly, is concerned with apparatus for detecting the onset of slag entrainment in a stream of molten steel such as occurs in the continuous casting process as a ladle empties into a tundish.
2. Description of the Prior Art
Continuous casting of steel billets, blooms and slabs is a process that converts batches of molten steel prepared in furnaces into a continuous product. This process consists of several intervening steps in which the steel receives the final chemical treatment and is held ready for pouring into the casting mold. In the first step, the steel is poured from the furnace into a transfer ladle and subjected to compositional analysis and modification, while maintained at the appropriate superheat temperature required for casting. In the next step, the ladle is moved into position over a tundish and molten steel is transferred in a continuous stream via gravity feed through a slide gate valve in the bottom of the ladle to the tundish. The tundish, which holds a smaller volume of steel, is stationarily positioned spaced above the mold and includes nozzles for guiding steel into molds.
As the ladle empties, its discharge into the tundish is terminated, and another ladle is brought into position to keep the tundish replenished with molten steel. Each ladle of molten steel is referred to as a "heat", and many heats are required for one continuous casting run. In order to maintain high quality and a uniform cast product, it is important to maintain a uniform quality of molten steel in the succession of heats.
Slag in the molten steel can reduce the uniform quality of the product if the slag is allowed to flow into the tundish from the ladle. Slag, which consists of various oxides created in the furnace and ladle, has a lower density than steel and consequently floats on the surface of the molten steel. Parenthetically, its presence does serve a useful purpose in that it forms a floating insulative layer which helps to maintain the superheat temperature of the molten steel.
Since the molten steel is withdrawn through the gate valve at the bottom of the ladle, slag is kept from contaminating the tundish (and the finished product) since it floats on the top surface. This technique works until near the end of each heat, when slag tends to mix with steel due to vortexing effects created by the steel discharge. In order to minimize contamination, the level of steel in the ladle is monitored visually and the flow is terminated when it appears to be near the onset of vortexing (i.e. slag entrainment) in the discharge stream. Usually, but not always, the flow is terminated early, and valuable steel is subsequently scraped along with slag for recycling in the furnace. Ocassionally, flow is not terminated in time, and large volumes of slag are sucked into the discharge and into the tundish. Using the visual sighting method mentioned above, the process is strictly a matter of judgement based upon experience. The tendency is to maintain the quality of the finished product by cutting flow of uncontaminated steel. For a typical caster, the net worth of scraped steel from each heat can be several hundred-thousand to several million dollars annually.
Several devices for slag detection appear in the prior art. Representative of such prior art devices are the ones disclosed in Japanese Pat. Nos. 52-29800 and 57-56154 and the Slag Detector available from Voest-Alpine. In particular, the Japanese patents both disclose devices for detecting slag outflow from a vessel containing molten metal. The devices use a coil to detect the difference in conductivity between the molten metal and the slag. Japanese Pat. No. 52-29800 allows eddy currents to be generated within the flow of molten metal. The effluence of slag in the flow is reflected in a change in the coil's impedance which is converted to a voltage in a bridge circuit. In Japanese Pat. No. 57-56154, the impedance change due to slag is measured by an impedance measuring device and then run through a bandpass filter. The Voest-Alpine device uses a diffrrential coil mounted on the ladle shroud. The coil generates eddy currents in the casting stream. The difference in electrical conductivity is used to distinguish slag and liquid steel. The induction changes in the coil are sensed by a bridge, amplified by means of a carrier frequency amplifier, and then rectified.
While these prior art devices might operate satisfactorily under the limited range of conditions for which they were designed, another critical condition that typically is present in molten steel discharge appears not to have been addressed by these prior art devices. This condition is fluctuations in the flow velocity of the molten steel discharge that could cause false alarms by indicating the onset of slag entrainment when, in fact, no slag is present. Consequently, a need still exists for a suitable apparatus for detecting the onset of slag entrainment in the steel discharge from a ladle in a continuous caster.